1. Field of the Invention
The present invention relates to a method of and an apparatus for improving the quality of pictures displayed by a picture display apparatus such as a television set, a video projector, or the like.
2. Description of the Related Art
It is known in the art that the quality of pictures displayed by a picture display apparatus such as a television set, a video projector, or the like is reduced by flare. The flare is a phenomenon caused when light leaks from a bright area into a dark area due to light reflections and dispersions in lenses and on illuminated surfaces of a projecting tube or picture tube. The flare results in blurs at edges of displayed pictures across which the luminance difference is large, e.g., a boundary between white and black areas.
FIG. 1 shows an original picture from which a picture is projected onto a screen by a video projector. As shown in FIG. 1, the original picture has a central rectangular white area WT and a black area BL disposed therearound, with the luminance difference being large across an edge ED at the boundary between the white area WT and the black area BL. FIG. 1 also illustrates, below the original picture, a video signal (i.e., luminance signal) representing the original picture horizontally across a central portion thereof. When the original picture is projected onto the screen by the video projector, light leaks from the white area WT into the black area BL, blurring the edge ED. The flare thus caused tends to reduce the quality of the projected picture.
In order to eliminate the above flare, the video signal to be supplied to the video projector is generally digitally processed to correct the picture data out of edge blurring. Such a digital signal process is referred to as flare correction or flare compensation. FIG. 2 shows the concept of flare correction. FIG. 2 shows at (a) the waveform of the video signal of an original picture, which corresponds to the video signal of the original picture shown in FIG. 1. FIG. 2 shows at (b) the luminance distribution of a picture which is projected and displayed on the screen based on the video signal shown at (a) in FIG. 2. FIG. 2 shows at (c) the waveform of a video signal which is produced by effecting flare correction on the video signal shown at (a) in FIG. 2. FIG. 2 shows at (d) the luminance distribution of a picture which is projected and displayed on the screen based on the flare-corrected video signal shown at (c) in FIG. 2.
The picture projected onto the screen by the video projector based on the video signal shown at (a) has its edge blurred by flare as shown at (b) in FIG. 2. In order to correct the picture, the video signal shown at (a) may be corrected or compensated at its positive- and negative-going edges thereof to correct (inversely correct) these edges depending on the blurring at the edges, i.e., to emphasize these edges, as shown at (c). By thus correcting the video signal, it is possible to display the picture free of edge blurs on the screen as shown at (d).
One conventional picture quality improving apparatus disclosed in Japanese laid-open patent publication No. 61-296880 (JP, 61-296880, A) carries out the above flare correction based on a correction signal which is generated from a luminance signal among luminance and color signals (wide-band and narrow-band color signals) which are produced from R (Red), G (Green), and B (Blue) signals as primary color signals. Specific details of the disclosed picture quality improving apparatus will be described below.
FIG. 3 shows in block form the picture quality improving apparatus disclosed in JP, 61-296880, A. As shown in FIG. 3, the picture quality improving apparatus comprises A/D converters 101a to 101c, matrix circuit 102, compensation delay circuits 103a to 103c, correction signal generator 104, combiners 105a to 105c, and D/A converters 106a to 106c. A/D converters 101a to 101c are supplied respectively with luminance signal Y, wide-band color signal C1, and narrow-band color signal C2 which are produced from R, G, B signals by an inverse matrix circuit, not shown. A/D converters 101a to 101c convert the supplied analog signals into digital signals. A/D converter 101a supplies its digital output signal to correction signal generator 104 and also to matrix circuit 102. A/D converters 101b, 101c also supply their digital output signals to matrix circuit 102.
Correction signal generator 104 comprises correction signal generating circuit 106 and delay unit 107 which are supplied with the luminance signal supplied from A/D converter 101a, and gain adjusting circuit 108 which is supplied with output signals from correction signal generating circuit 106 and delay unit 107.
Matrix circuit 102 converts luminance signal Y, wide-band color signal C1, and narrow-band color signal C2 supplied respectively from A/D converters 101a, 101b, 101c into R, G, B signals as primary color signals. The R, G, B signals output from matrix circuit 102 are supplied respectively to compensation delay circuits 103a to 103c. 
Combiner 105a has an input terminal supplied with the output signal from compensation delay circuit 103a and another input terminal supplied with the output signal from gain adjusting circuit 108. Combiner 105a outputs a signal which is a combination of the supplied signals to D/A converter 106a. Similarly, combiner 105b is supplied with the output signal from compensation delay circuit 103b and the output signal from gain adjusting circuit 108, and outputs a signal which is a combination of the supplied signals to D/A converter 106b. Combiner 105c is supplied with the output signal from compensation delay circuit 103c and the output signal from gain adjusting circuit 108, and outputs a signal which is a combination of the supplied signals to D/A converter 106c. 
The picture quality improving apparatus shown in FIG. 3 operates as follows: When a digital luminance signal converted by A/D converter 101a is supplied to correction signal generator 104, correction signal generating circuit 106 filters the supplied luminance signal to generate a correction signal. Usually, a picture display apparatus such as a television set, a video projector, or the like is supplied with a nonlinear input signal multiplied by a gamma value because of the characteristics thereof, e.g., the characteristics of cathode-ray tubes (CRTs). If such a nonlinear input signal is filtered into a flare correction signal and such a flare correction signal is added, then the linearity of the flare correction signal itself is lost, lowering the sensitivity of the correction filter in dark picture areas where the signal level is low, with the result that the picture quality cannot sufficiently be improved in such dark picture areas. In order to avoid the above drawback, gain adjusting circuit 108 adjusts the gain of the correction signal generated by correction signal generating circuit 106.
The correction signal whose gain has been adjusted by gain adjusting circuit 108 is supplied to combiners 105a to 105c. Combiners 105a to 105c combine the respective R, G, B signals, which have been delayed by respective compensation delay circuits 103a to 103c for a time equal to the delay time required for correction signal generating circuit 106 to generate the correction signal, with the correction signal whose gain has been adjusted by gain adjusting circuit 108, thus effecting flare correction on the R, G, B signals.
Internal details of correction signal generating circuit 106 are shown in FIG. 4. The picture quality improving apparatus also performs a contour emphasis process for emphasizing the contour of a picture in order to prevent the resolution from being lowered. The contour emphasis process and the flare correction process are concurrently performed in correction signal generating circuit 106. Correction signal generating circuit 106 has a compensation delay unit 125 for generating and delaying a signal 1 for a contour correction filter system, a signal m for a flare correction filter system, and a reference signal n, from the input signal applied to correction signal generating circuit 106. The contour correction filter system comprises vertical contour correction FIR (Finite Impulse Response) filter 121 and horizontal contour correction FIR filter 122 which are connected in cascade, subtractor 126a for subtracting a filtered signal from the reference signal n, and coring circuit 127a connected to the output terminal of subtractor 126a. The flare correction filter system comprises vertical flare correction composite IIR (Infinite Impulse Response) filter 123 and horizontal flare correction composite IIR filter 124 which are connected in cascade, subtractor 126b for subtracting a filtered signal from the reference signal n, and coring circuit 127b connected to the output terminal of subtractor 126b. Correction signal generating circuit 106 also has an adder 128 for adding the output signals from coring circuits 127a, 127b to each other and outputting the sum signal as the output signal from correction signal generating circuit 106.
Since the present invention is concerned with improving the picture quality based on the flare correction, details of correction signal generating circuit 106 for carrying out the flare correction will be described below. Vertical flare correction composite IIR filter 123 and horizontal flare correction composite IIR filter 124 which are connected in cascade jointly provide a two-dimensional (2D) low-pass filter (LPF). FIG. 5 shows vertical flare correction composite IIR filter 123 in block form, and FIG. 6 shows horizontal flare correction composite IIR filter 124 in block form.
As shown in FIG. 5, vertical flare correction composite IIR filter 123 comprises two low-pass IIR filters 230a, 230b, and two field inverters 234a, 234b. Two low-pass IIR filters 230a, 230b are identical in structure to each other, and a specific circuit arrangement of filter 230a among two low-pass IIR filters 230a, 230b is shown in detail.
Low-pass IIR filter 230a comprises delay elements 231a to 231c which comprise line memories, coefficient circuits 232a to 232d connected respectively to taps and an input terminal, and adders 233a to 233c. Coefficient circuit 232d is supplied with the luminance signal from A/D converter 101a and supplies its output signal to an input terminal of adder 233a. Adder 233a supplies its output signal to field inverter 234a. The output signal from adder 233a is also supplied via delay elements 231a to 231c, which are connected in cascade, to coefficient circuits 232a to 232c which are supplied with the output signals from delay elements 231a to 231c. The output signals from coefficient circuits 232a to 232c are added to each other by adders 233b, 233c, and the sum signal is supplied to the other input terminal of adder 233a. Low-pass IIR filter 230a thus arranged serve as a recursive filter.
In vertical flare correction composite IIR filter 123, the output signal from low-pass IIR filter 230a is inverted in each field by field inverter 234a, which supplies the inverted output signal to low-pass IIR filter 230b that is structurally identical to low-pass IIR filter 230a. The output signal from low-pass IIR filter 230b is inverted in each field by field inverter 234b. The delay in phase caused by low-pass IIR filter 230a is compensated for because the signal is advanced in phase by low-pass IIR filter 230b when the inverted signal is applied thereto. The filter arrangement shown in FIG. 5 provides a good low-pass filter.
As shown in FIG. 6, horizontal flare correction IIR filter 124 comprises two low-pass IIR filters 240a, 240b, and two line inverters 244a, 244b. Horizontal flare correction IIR filter 124 is of basically the same structure as vertical flare correction composite IIR filter 123 shown in FIG. 5, but differs therefrom in that line inverters 244a, 244b are used in place of field inverters 234a, 234b. Two low-pass IIR filters 240a, 240b are identical in structure to each other, and each comprises delay elements 241a to 241c which comprise A/D conversion clock registers, coefficient circuits 242a to 242d connected respectively to taps and an input terminal, and adders 243a to 243c. Low-pass IIR filters 240a, 240b are of basically the same structure as low-pass IIR filters 230a, 230b shown in FIG. 5 except that delay elements 241a to 241c are used in place of delay elements 231a to 231c which comprise line memories.
In addition to JP, 61-87493, A, further picture quality improving apparatuses for carrying out the flare correction and the contour emphasis concurrently with each other are disclosed in JP, 61-87493, A; JP, 61-88663, A; JP, 61-88664, A; JP, 61-88665, A; JP, 61-88667, A; JP, 61-88668, A; JP-88669, A; JP, 61-270987, A; JP, 61-270991, A; JP, 61-270992, A; JP, 61-270993, A; JP, 61-270994, A; JP, 61-270995, A; JP, 61-295786, A; JP, 61-295791, A; JP, 61-295793, A; JP, 61-296879, A; JP, 61-296881, A; JP-296883, A; JP, 61-296884, A. A picture quality improving apparatus which performs the flare correction using a cyclic or acyclic filter is disclosed in JP, 1-246985, A.
In the picture quality improving apparatus disclosed in the publications JP, 61-296880, A, etc., each of vertical flare correction composite IIR filter 123 (FIG. 5) and horizontal flare correction composite IIR filter 124 (FIG. 6) is of an arrangement for inverting data and requires two frame memories. Therefore, these picture quality improving apparatus cannot easily be reduced in cost and size.
The picture quality improving apparatus disclosed in the publications JP, 61-296880, A, etc. employ a 2D LPF which is made up of the vertical flare correction composite IIR filter and horizontal flare correction composite IIR filter that are connected in cascade. However, there are other picture quality improving apparatus which employ composite FIR filters for vertical and horizontal flare correction. These other picture quality improving apparatus are disadvantageous in that their circuit scale is large though no frame memory is required. For example, the vertical flare correction composite FIR filter has an increased number of multipliers and is large in circuit scale as a delay caused for each line is multiplied as a coefficient.
It is known in the art that the visual effects of contour and contrast can be increased by using the Craik-O'Brien effect with respect to visual perception. There have not been known in the art any arrangements which apply the Craik-O'Brien effect to the flare correction. The Craik-O'Brien effect is also known as Craik-Cornsweet illusion, and will be described in detail later on.
The aspect ratio, that is, ratio of frame height to frame width, of picture display apparatus such as a television set has heretofore been 3:4. With the growing popularity of digital high-definition picture contents, picture display apparatus having a screen whose aspect ratio is 9:16 are becoming more and more general. A screen having such an aspect ratio is referred to as “wide screen”. However, the conventional picture quality improving apparatus described above are not arranged to be compatible with such picture display apparatus having an aspect ratio of 9:16, and cause the following problems if applied to the picture display apparatus having an aspect ratio of 9:16:
When a picture having an aspect ratio of 3:4 is displayed on a picture display apparatus having a wide screen whose aspect ratio is 9:16, an edge is produced at the boundary between a black area and an effective area of the picture, reducing the quality of the picture. If a picture having an aspect ratio of 3:4 is expanded horizontally or both horizontally and vertically in order to be displayed on the wide screen, then the desired improved effect (flare correction or Craik-O'Brien effect) may not be achieved. If a picture having an aspect ratio of 3:4 is expanded nonlinearly in order to be displayed on the wide screen, then the desired improved effect (flare correction or Craik-O'Brien effect) may not be achieved by expanding the picture after the flare correction has been performed.